Investigations in non-human primates have demonstrated a circadian pattern of change in cerebrospinal fluid prolactin concentrations similar to that previously observed in plasma. Other neuroendocrine substances (e.g., cortisol and beta-endorphin) as well as amines and amine metabolites are being studied in relation to neurotransmitter-related drug effects. Measurement of these same substances in cat and rodent cerebrospinal fluid and brain supplement the studies done in monkeys and man. Evidence has been obtained that the neurotransmitter changes induced by the selective monoamine oxidase type A inhibitor clorgyline which has antidepressant properties leads to decreases in the numbers of adrenergic receptors in rat brain. Pargyline, an ineffective antidepressant in our human studies, does not. The temporal pattern of the clorgyline-induced changes in clonidine binding suggests that alterations in presynaptic mechanisms precede those in postsynaptic sites.